The large SV40 T antigen is a DNA binding phosphoprotein. It may also be closely associated with a protein kinase activity and as such has the capacity to phosphorylate itself. The function of this antigen is necessary for SV40 induced neoplastic transformation, the initiation of SV40 DNA replication, and the control of early viral transcription. Large T purified from SV80 cells, an SV40-transformed line, binds preferentially to certain sites on the SV40 genome. I would first like to investigate whether the state of T phosphorylation affects these DNA binding properties. Specifically, I should like to determine whether the state of large T antigen phosphorylation affects both its quantitative ability to bind to various DNAs as measured by the nitrocellulose filter binding technique and also whether binding to certain specific sites on the SV40 genome is detected by nuclease protection experiments. Secondly, there is immunofluorescent banding evidence which suggests that T antigen associates with mammalian chromosomes in a site specific manner. To pursue this possibility further, I wish to ascertain, directly, whether preferential binding sites for SV40 large T antigen exist in cellular DNA and, if so, to determine the nucleotide sequences of one or more of these loci. To answer these questions, I shall use a cloned 'library' of shotgun mouse cellular DNA as well as certain well-characterized cloned mouse genes. Using various techniques involving protection of selected sites from digestion by a specific nuclease, I shall attempt to identify and characterize any specific T antigen binding sequences which may exist.